This invention is generally directed to processes for the preparation of superconductors, and more specifically the present invention is directed to processes for preparing pastes that may be selected as precursors for the preparation of superconductors, including certain high temperature copper oxide superconductors. Thus, in accordance with the simple, economical process of the present invention there can be prepared superconductors of, for example, the formula YBa.sub.2 Cu.sub.3 O.sub.7-x, wherein x is close to zero by the mixing and grinding of yttrium nitrate hydrate, an oxidizing agent, and copper nitride, adding a solvent thereto such as acetone, forming a paste thereof, heating the paste, and thereafter cooling. One specific embodiment of the present invention comprises the mixing of yttrium nitrate hydrate an oxidizing agent, such as barium peroxide, and copper nitride; grinding the aforementioned mixture; adding to the mixture a solvent such as acetone; grinding the formed mixture to enable the formation of a paste; applying the paste to a substrate such as a ceramic alumina rod; heating the rod with the paste thereon; and subsequently cooling the rod whereby there results thereon a copper oxide superconductor. With the process of the present invention, there is provided a superconductor of high purity, exceeding 95 percent in some instances, wherein only a single heating step is needed. Moreover, with the process of the present invention a paste is formed enabling it to be premanently coated on various substrates, such as ceramic rods. Typically, superconductor precursor materials for the formation of YBa.sub.2 Cu.sub.3 O.sub.7-x and other copper oxide superconductors are drypowders that do not form adherent coatings on substrates prior to firing, or that do not retain their form or shape prior to firing unless compressed under high pressure or placed in a mold. A paste, however, which is formed with the process of the present invention, and whose conistency and thickness may be varied, can be selected for the formation of a superconductor precursor Thus, for example, when it is desirable to coat a ceramic rod, such as an alumina rod with a copper oxide superconductor, such as YBa.sub.2 Cu.sub.3 O.sub.7-x, this may not be accomplished simply by dipping the rod into the dry powder percursor mixture as the powder will not usually adhere uniformly to the rod surface. With an adherent paste obtained with the process of the present invention, ceramic rods can be uniformly coated by dipping the rod into the paste and heating at a temperature of from about 900.degree. to about 975.degree. C. During the heating process, the precursor paste converts to a superconductor such as YBa.sub.2 Cu.sub.3 O.sub.7-x which is uniformly coated on the rod. In a similar manner, the paste formed with the process of the present invention may be applied to other substrates to which it adheres resulting, for example, in thick films (greater than 0.1 micrometer) of superconductors. Since the paste is adherent, the substrate may assume any attitude during the firing or heating process without removal of the precursor as is the siuation with dry powders. Also, the paste obtained with the process of the present invention may be used to form wires or electrically conducting paths on a substrate by extruding the paste through a fine orifice. Thus, a circuit board may be formed on an appropriate substrate, such as alumina or silicon, that would consist of superconducting wires. Furthermore, superconducting spheres may be formed by rolling and shaping a paste having the consistency of modeling clay into the desired shape and heating the paste to a temperature of from about 900.degree. to about 975.degree. C.
Processes for the preparation of superconductors are known. Thus, for example, it is known that copper oxide superconductors can be prepared by high temperature, above 950.degree. C., ceramic methods. In the aforementioned processes barium carbonate, Y.sub.2 O.sub.3, and copper oxide are admixed, ground with a motar and pestle, and subsequently the resulting product is pressed into pellets. Thereafter, the products are fired in an oven at about 950.degree. C. for about 10 hours. The pellet product after cooling is ground, and pressed a second time, and thereafter sintered at 950.degree. C. for in excess of 10 hours. The resulting material shows a variety of superconducting properties such as zero dc resistance and the Meissner effect. Often the material is multiphased, however, with nonsuperconducting phases diluting the superconductor phase. The copper oxide superconductor resulting of the formula illustrated herein usually contains therein impurities such as nonsuperconducting phases, reference for example the Japanese Journal Of Applied Physics, Volume 26, No. 5, May of 1987, and the Japanese Journal of Applied Physics, Volume 26, No. 4, Apr. 1987, pages L498 to L501. For example, one specific contaminant present in the aforementioned superconductors is believed to be a green compound Y.sub.2 BaCuO.sub.5, which in of itself does not function as a superconductor.
In Chapters 7 and 11 of the Chemistry of High-Temperature Superconductors, American Chemical Society, Davision et al., there is disclosed the preparation of superconductors of the formula YBa.sub.2 Cu.sub.3 O.sub.7 by dissolving stoichiometric amounts of yttrium oxide, copper oxide, and barium carbonate in concentrated nitric acid. Thereafter, the solution formed is evaporated to dryness, followed by decomposition of the nitrate mixture at 500.degree. C. The resultant grey material was then grounded mechanically and refined at 750.degree. C., and subsequently the grinding and heating steps were repeated at 900.degree. to 950.degree. C. A final annealing step was then accomplished by heating the product to 950.degree. C. in pure oxygen.
In copending application U.S. Ser. No. 188,889, the disclosure of which is totally incorporated herein by reference, there are illustrated processes for the preparation of copper oxide superconductors by mixing copper nitride, an oxidizing agent, and yttrium oxide; forming pellets of the aforementioned mixture; heating the pellets at a temperature of from about 900.degree. to about 975.degree. C.; and thereafter cooling said pellets.
Accordingly, while processes for the preparation of superconductors are known, there is a need for further simple, economical processes that will enable copper oxide superconductors of a high purity. More specifically, there is a need for processes that will enable the formulation of copper oxide superconductors of a purity of 80 percent, or greater, from pastes. There is also a need for efficient processes for the preparation of copper oxide superconductors from paste precusors formed by mixing stoichiometric amounts of yttrium nitrate hydrate, an oxidizing agent, and copper nitride. Also, there is a need for processes wherein high purity superconductors, exceeding 95 percent, and as high as 100 percent in some instances can be obtained with one heating step, and the product resulting has improved homogeneity.